The present invention relates to the art of diagnostic medical imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI) or MRI scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
The medical field has found MRI to be a valuable diagnostic tool for the non-invasive study or examination of a patient""s anatomy. Various MRI scanners and apparatus have been describe in detail in the prior art. As is known in the art, by applying a particular combination of radio frequency (RF) pulses and magnetic gradients to a spin system set up in a region of interest, a signal therefrom (often comprising a plurality of echos) can be induced, received and processed into an image representation of the region of interest. The combination of RF pulses and magnetic gradients is commonly referred to as the pulse sequence. Again, various well known pulse sequences have been developed to address specific imaging needs.
Generally, in MRI, a substantially uniform temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged or examined is placed. Nuclei in the subject have spins which in the presence of the main magnetic field produce a net magnetization. The nuclei of the spin system precess in the magnetic field at the Larmor frequency, i.e., the resonant frequency. Radio frequency (RF) magnetic fields at and/or near the resonant frequency are used to manipulate the net magnetization of the spin system. Among other things, RF magnetic fields at the resonant frequency are used to, at least partially, tip the net magnetization from alignment with the main magnetic field into a plane transverse thereto. This is known as excitation, and the excited spins produce a magnetic field, at the resonant frequency, that is in turn observed by a receiver system. Shaped RF pulses applied in conjunction with gradient magnetic fields are used to manipulate magnetization in selected regions of the subject and produce a magnetic resonance (MR) signal. The resultant MR signal may be further manipulated through additional RF and/or gradient field manipulations to produce a series of echos (i.e., an echo train) as the signal decays. The various echos making up the MRI signal are typically encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data line""s position in k-space (i.e., its relative k-space row) is typically determined by its gradient encoding. Ultimately, in an imaging experiment, by employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space (or reciprocal space) data.
One well established pulse sequence and MRI technique is echo planar imaging (EPI). EPI is a general method of imaging which can be performed with, what is known as, either spin-echo, field-echo or stimulated-echo contrast. EPI is well suited to rapid imaging applications, e.g., real-time cardiac cine imaging, cardiac first pass perfusion imaging, and general interactive scanning procedures.
The speed of the EPI technique can be attributed to the fact that the signal is refocused into multiple echos by rapid reversals of the polarity of the read gradient. Typically, phase encoding of these echos forms data lines in the k-space of a final image. A data set is often built up using only a small number of excitations, or shots. In extreme cases, an image is obtained in a single shot. Given favorable system capabilities, multi-shot images can be generated in times as short as 50 to 100 ms. That equates to a rate of about 10 to 20 images per second, i.e., video rate image acquisition.
However, EPI does have certain disadvantages. For example, EPI is sensitive to field distortion and phase ghosting artifacts. Field distortion artifacts are caused by the low bandwidth per pixel employed in the phase encoding direction, while ghosting artifacts are caused by combining time reversed copies of data lines obtained under negative read gradient lobes with non-reversed data lines obtained under positive read gradient lobes. Small differences in the refocusing of the two types of data lines caused by hardware imperfections and spin evolution during the echo train are often the main sources of ghosting artifacts.
Interleaving the data lines from a small number of excitations is known to be effective at reducing the sensitivity to field distortion artifacts for only a small cost in data collection efficiency. Interleaving, however, does not solve the problem of ghosting. Ghosting is often addressed by using correction scans, sliding window acquisition or corrective reconstruction procedures that compensate for intrinsic differences between the signals generated in the echo train.
The present invention contemplates a new and improved EPI fluoroscopic technique which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of imaging a subject with an MRI scanner is provided. The method includes positioning a region of interest of the subject in the MRI scanner and executing an echo planar imaging pulse sequence therewith. A signal produced by the echo planar imaging pulse sequence is received. The signal includes first and second distinct sets of echos. Each echo in the first set of echos coincides with a positive polarity read gradient of the echo planar imaging pulse sequence and each echo in the second set of echos coincides with a negative polarity read gradient of the echo planar imaging pulse sequence. The method further includes collecting into a first k-space matrix data corresponding to the first set of echos and collecting into a second k-space matrix data corresponding to the second set of echos. A first image of the region of interest is reconstructed from the data in the first k-space matrix, and a second image of the region of interest is reconstructed from the data in the second k-space matrix.
In accordance with a more limited aspect of the present invention, the first and second images of the region of interest are temporally offset from one another.
In accordance with a more limited aspect of the present invention, the reconstructing steps are carried out employing a Fourier transformation.
In accordance with a more limited aspect of the present invention, the reconstructing steps are carried out employing a partial Fourier transformation.
In accordance with a more limited aspect of the present invention, the echo planar imaging pulse sequence is a multi-shot sequence including a plurality of excitations.
In accordance with a more limited aspect of the present invention, the first and second k-space matrices each include a plurality of interleaves which are successively filled with data.
In accordance with a more limited aspect of the present invention, the steps are iteratively carried out to achieve cine or fluoroscopic imaging of the region of interest.
In accordance with another aspect of the present invention, a magnetic resonance imaging apparatus includes an MRI scanner in which a region of interest of a patient is positioned. The MRI scanner is operable to carry out an echo planar imaging pulse sequence. A receiver observes a signal produced by the echo planar imaging pulse sequence. The signal including first and second distinct sets of echos. Each echo in the first set of echos coincides with a positive polarity read gradient of the echo planar imaging pulse sequence and each echo in the second set of echos coincides with a negative polarity read gradient of the echo planar imaging pulse sequence. Included sorting means route into a first k-space matrix data corresponding to the first set of echos and route into a second k-space matrix data corresponding to the second set of echos. A reconstruction processor reconstructs a first image of the region of interest from the data in the first k-space matrix and reconstructs a second image of the region of interest from the data in the second k-space matrix.
In accordance with a more limited aspect of the present invention, the first and second images of the region of interest are temporally offset from one another.
In accordance with a more limited aspect of the present invention, the reconstruction processor employs a Fourier transformation.
In accordance with a more limited aspect of the present invention, the reconstruction processor employs a partial Fourier transformation.
In accordance with a more limited aspect of the present invention, the echo planar imaging pulse sequence is a multi-shot sequence including a plurality of excitations.
In accordance with a more limited aspect of the present invention, the first and second k-space matrices each include a plurality of interleaves which are successively filled with data.
In accordance with a more limited aspect of the present invention, cine or fluoroscopic imaging of the region of interest is achieved via iterative operation of the magnetic resonance imaging apparatus.
In accordance with a more limited aspect of the present invention, the magnetic resonance imaging apparatus further includes a human viewable display which alternately displays the first and second images from each iteration.
In accordance with another aspect of the present invention, a method of imaging a patient with an MRI scanner includes positioning a region of interest of the patient in the MRI scanner and executing a echo planar imaging pulse sequence therewith. A signal produced by the echo planar imaging pulse sequence is received. The signal includes a plurality of echos, each of which coincides with a positive or negative polarity read gradient of the echo planar imaging pulse sequence. The method further includes sampling each echo to acquired data therefrom and sorting the acquired data into a plurality of distinct k-space matrices such that all the data in any one k-space matrix is acquired from echos coinciding with read gradients of like polarities. Finally, a plurality of distinct images of the region of interest are reconstructed from corresponding data in each of the k-space matrices. The plurality of distinct images are each temporally offset from one another.
In accordance with a more limited aspect of the present invention, the reconstructing is carried out employing a full Fourier transformation or a partial Fourier transformation.
In accordance with a more limited aspect of the present invention, the echo planar imaging pulse sequence is a multi-shot sequence including a plurality of excitations.
In accordance with a more limited aspect of the present invention, the k-space matrices each include a plurality of interleaves which are successively filled with data.
In accordance with a more limited aspect of the present invention, the steps are iteratively carried out to achieve cine or fluoroscopic imaging of the region of interest.
One advantage of the present invention is ghost reduced cine or fluoroscopic MRI imaging.
Another advantage of the present invention is data acquisition efficiency.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.